The identity of DM is still unknown, and one plausible explanation is that it is composed of unknown particles. Various candidates for particle DM have been proposed, and one of the most promising ones is the axion [1]}, [2]}, [3]}, [4]}. The axion is a pseudo Nambu-Goldstone (NG) boson that appears due to spontaneous breaking of the global U(1) symmetry.
As such it can easily satisfy the stability requirement for DM because it has naturally a light mass and very weak interactions with the SM particles, if the symmetry breaking scale is sufficiently high. Because of its light mass, the axion is expected to be generally away from its potential minimum in the early universe, so it starts oscillating around the potential minimum when the Hubble parameter becomes roughly equal to the curvature of the potential. The oscillation energy is a natural explanation for the cold axion DM. This is known as the misalignment mechanism [5]}, [6]}, [7]}. The advantage of the axion DM is that it can naturally explain the stability and generation of DM. See Refs. [8]}, [9]}, [10]}, [11]}, [12]}, [13]}, [14]} for reviews.
